The present invention relates to systems for reproducing sound in general and to systems which can control sound production to localized regions in particular.
A typical sound system performs two functions: amplifying sound and reproducing sound with a given level of clarity or intelligibility within a particular room, auditorium, hall or other space. Sound may be either audio frequency sound, subsonic sound, or ultrasonic sound. The audio frequency sound falls within the range of 15 Hz to 20,000 Hz, the range generally of human hearing, with subsonic frequencies being those below 15 Hz, and ultrasonic frequencies being those above 20,000 Hz.
Recently new capabilities have led to research in sound systems which could have the potential to produce sound which is contained within a beam, or which is aimed at a particular point or listener. Such systems open up the possibility of providing different audio stimulus to different people occupying the same room, museum, or lecture hall. Such a system might also provide more realistic stereo without using headphones by providing a separate audio input to each ear of a listener.
One approach proposed by Joe Pompei while a student at MIT, involves generation of ultrasonic sound which distorts in a predictable way so that the distortions produce audio frequency sound. Starting with the desired audio frequency sound it is mathematically possible to predict the ultrasonic beam which will produce the desired audio frequency sound. By such means Pompei is able to generate an audio spotlight of sound.
Another proposed approach is to use an acoustic time-reversal mirror. Such systems have been developed by Mathias Fink at Ecole Superieure de Physique et de Chimie Industrielles de la Ville de Paris. A time-reversal mirror is a concept known from optics where it is known to be possible to construct a mirror which sends light reflected therefrom directly backwards so that the lightwaves appear to be reversed in time. Thus light emitted from a point when reflected in a phase conjugate, or time-reversal mirror, returns to the emitting point. To look into a time-reversal mirror is to see only the light emitted from the pupil which is gazing into the mirror. In a similar way, an acoustic time reversal mirror returns sound to the source that emitted the sound. This returning sound returns identically to that emitted, even if the path between the sound source and the time-reversal mirror involves many reflections, distortions, and dispersions. At least in theory, the time reversal process could be used to focus sound at a particular location so that different sounds would be heard by different people.
A wide variety of audio systems attempt to provide more realistic sound by providing an array of speakers which produce the effect that the sound appears to come from a particular direction or source and such systems are described in U.S. Pat. No. 5,521,981 to Gehring or U.S. Pat. No. 5,974,152 to Fujinami. More generally, any stereo, quadraphonic, or surround sound system uses multiple speakers to produce sound which is more realistic.
However, none of the foregoing systems has produced a cost-effective system for providing sound which can only be heard in a localized region. What is needed is an apparatus and method for producing audible sounds which are localized so that multiple listeners can be provided with unique audio input without the use of headphones.